1. Field of the Invention
The present invention is broadly concerned with an improved, filled synthetic resin insulator body especially adapted for use in the context of electrical cutout devices and being characterized by enhanced arc track resistance and beam strength and the ability to employ embedded mounting hardware in the insulator body. More particularly, it is concerned with such an improved insulator, and a complete cutout device, wherein the insulator is formed of a cured epoxy resin matrix filled with a quantity of glass beads. Proper selection of the size and quantity of glass beads gives a desirably low viscosity to the resin system in an uncured state, while increasing the requisite electrical and strength properties of the resin system upon curing. The insulator is also desirably formed with an oval-like cross section to present a greater effective width than thickness, and with a longitudinal bow to give an arcuate central axis; this construction not only increases the mechanical beam strength, but maximizes the flashover distance of a completed cutout device.
2. Description of the Prior Art
Conventional electrical cutouts used in transmission and distribution systems broadly include an elongated, skirted insulator typically formed of porcelain, with a pair of endmost, laterally projecting, line-connecting metallic terminals. An elongated fuse assembly is mounted between and electrically couples the respective terminals, and is adapted to sever under the influence of a fault current in order to interrupt the current.
While such cutouts have long been used, they present a number of problems, particularly insofar as the insulator thereof are concerned. In particular, most prior cutout insulators are formed of porcelain. As a consequence, these insulators are without known exception configured as surfaces of revolution, e.g., they are of circular cross section throughout. This stems from the fact that virtually insuperable difficulties are presented in the fabrication of porcelain insulators having shapes other than surfaces of revolution. This manufacturing constraint in turn means that prior porcelain insulators cannot be formed with the most advantageous shape from the standpoint of structural intergrity. That is to say, when a cutout device operates, there can be substantial, potentially destructive forces imposed on the insulator. In order to absorb these relatively high magnitude forces, an insulator must possess the requisite degree of beam strength. If, however, it is only possible to fabricate an insulator as a surface of revolution, such an insulator must have a relatively large diameter to possess the necessary beam strength. On the other hand, if it were possible to fabricate such an insulator as a beam-like shape having a greater width than thickness, then maximum beam strength could be obtained while using a minimum of material. Thus, porcelain insulators are of necessity inefficient from a structural/mechanical strength point of view, because of the inability to fabricate porcelain as an insulator having the most optimum shape.
In addition, because of the use of porcelain in cutout insulators, it has been necessary to attach the terminals and mounting hardware as structurally distinct items, as opposed to being embedded in the porcelain itself. Prior expedients used in this context have included attaching the metallic end terminals and central mounting hardware by means of bands secured to the insulator body, or through the use of conventional sulfur cements. These techniques are troublesome not only because of the extra manufacturing steps involved, but also by virtue of the fact that the electrical characteristics of the overall cutout may be compromised when using such externally mounted hardeware.
In view of the forgoing difficulties, it has been proposed in the past to make use of synthetic resin materials in the fabrication of cutout insulators. For example, polyester and expoxy systems filled with hydrated alumina have been proposed for the manufacture of such insulators, see U.S. Pats. Nos. 2,961,518, 4,206,066, 3,511,922 and 3,838,055. However, these prior synthetic resin insulators have typically suffered from manufacturing difficulties (e.g., the material in an uncured state is extraordinarily viscous, making it difficult to cast), low mechanical strength, or relatively low arc track resistance in use. Accordingly, synthetic resin insulators have achived only moderate commercial success, and porcelain insulators are still in widespread use notwithstanding their inherent problems.